I think you're missing the point. When they say that life is a gift to be appreciated, they also imply that it is wonderful and necessarily worth living for. For someone to have a genuinely miserable life is just an oxymoron with this logic and these "miserable" people should really learn to appreciate what they've got more.
For example, while it's always morally awkward to deal with people who scarcely have enough to eat, if you were to take one of those annoying emo teenagers who commits suicide because his favorite band stopped playing or something silly like that, you'd agree that their misery is their own fault, right? And that they should appreciate what they do have.
With this worldview, you're just extending that to everyone who claims to be miserable, including the chronically poor, saying that a lack of material wealth should not stop them from appreciating the gift of life and that they still ought to thank God that they at least got to live.
“These are my principles; if you don’t like them, I have others.” – Groucho Marx
